1. Technical Field of the Invention
The present invention relates generally to dental X-ray systems, and more particularly to dental X-ray systems providing for alignment of the X-ray cone with an X-ray film plate.
2. Description of the Prior Art
Dental X-my photography has become an invaluable diagnostic aid to the dentist and today is often routinely performed in the dental office. However, difficulties are encountered in obtaining proper alignment of the dental X-ray apparatus with an X-ray film plate because the film plate is positioned intraorally and therefore cannot be seen by the dentist or technician during the aiming process. To avoid the problem of misalignment, a larger X-ray beam size is often used. This assures that the complete film plate will be exposed; however, it also increases the hazard caused by exposure of the patient to larger amounts of X-ray radiation.
Various X-ray alignment devices appear in the prior art which obtain proper positioning by mechanical means. U.S. Pat. No. 3,930,164 to Alexander discloses one such device that uses a centering rod to locate the center of the X-ray beam path and to position the subject accordingly. U.S. Pat. No. 4,057,733 to Hofmockel discloses an X-ray image intensifier which is mechanically mounted to the X-ray apparatus in such a manner that it can receive the emitted X-ray beam. The X-ray image received is reproduced on a video screen. In German Offenlegungsschrift No. 2,134,106 to Graf there is disclosed an X-ray film plate which is positioned on a frame mounted to the X-ray apparatus. These and similar devices using mechanical mounting means to obtain alignment have several disadvantages in that they are uncomfortable to the patient; they are inconvenient to use and difficult to position within the mouth; and they also pose a problem of sterilization. Further, the Alexander device is not adaptable for use in X-raying individual teeth within the mouth. Because the Hofmockel device uses a continuous beam, the overall X-ray exposure dosage to the patient is increased.
There are several prior art devices which use optics in order to obtain the desired positioning. U.S. Pat. No. 4,060,733 to Franke and U.S. Pat. No. 5,068,887 to Hughes both disclose a light beam which is reflected to illuminate the area at which the X-ray machine is pointed. In the field of dental X-rays, however, these are unsatisfactory devices because with dental X-rays, neither the film position nor the teeth are typically exposed to view. U.S. Pat. No. 3,790,803 to Phillips discloses another device that uses optics to obtain proper alignment. In Phillips, a beam of light is emitted from a fixed position on the X-ray apparatus. The beam of light is reflected from a fixed position relative to the X-ray film plate. In this manner, the device can indicate when there is proper alignment of the X-ray apparatus with the X-ray film plate. U.S. Pat. No. 4,012,638 to Altschuler discloses a device which applies optics specifically to the field of dental X-ray alignment. In Altschuler a plurality of infrared emitters and detectors are positioned on the X-ray apparatus. The X-ray film plate is positioned on a frame which extends outside the mouth and has an infrared reflective surface. When the infrared light emitted is reflected back and detected, alignment is indicated. Altschuler possesses many of the disadvantages of the mechanically mounted alignment devices, in that the film plate necessarily has to extend outside of the mouth. This attribute presents the problems of discomfort to the patient and of difficulty in positioning the film plate in its desired location within the mouth. The Altschuler device has the further disadvantage in that it does not determine the distance between the X-ray apparatus and the X-ray film plate. This information is desirable to properly adjust the size of the X-ray beam and therefore limit the amount of X-ray exposure to the patient. This distance is typically fixed in the mechanically mounted devices.
U.S. Pat. No. 4,223,228 to Kaplan discloses the prior art alignment system illustrated in FIG. 1. The X-ray source 10 includes a series of Hall effect sensors 11-14A and 11-14B on the X-ray cone. The X-ray film plate 43 to be exposed is placed within the patient's mouth 40 and held in place between teeth 44 by a tab 45. On the back of the X-ray film plate 43, there is a container 42 which holds a permanent magnet 41. The Hall effect sensors in the X-ray cone sense the magnetic field produced by the magnet 41. The signals from the Hall effect sensors are provided to a measurement and display means 30 via signal line 5. The display 32 includes a series of horizontal LEDs 33 and vertical LEDs 34, intersecting at the LED 37. The signals from the Hall effect sensors are processed such that illumination of the LEDs 33, 34 indicate the displacement of the X-ray source 10 relative to the film plate 43, with illumination of the LED 37 being the preferred position. The sensors 11B-14B are positioned far enough away from the magnet 41 so as to be unaffected by its magnetic field. Therefore, subtraction of the signals 11B-14B from the respective signals 11A-14A, correct for any effects of the Earth's magnetic field.
Although the X-ray alignment system of FIG. 1 represented a significant advance in the art at the time of invention, it nevertheless suffers from several disadvantages: the sensitivity of the Hall effect sensors impairs alignment when the X-ray cone is more than 31/2 inches from the magnet; the low signal-to-noise ratio of the Hall effect sensors causes distortion error; the magnet 41 does produce effects on the sensors 11B-14B, thereby reducing the effectiveness of the correction for the Earth's magnetic field; while position of the X-ray film plate can be aligned with the X-ray cone, the angular orientation of the film plate relative to the X-ray cone cannot be measured or corrected; and the LED display is difficult for an operator to use and does not allow for display of angular orientation error. There is therefore a need in the prior art for a dental X-ray alignment system which overcomes any or all of these limitations. The present invention is directed towards meeting those needs.